Re: [PATCH] tracing/hwlat: Bind the sampler kthread instead of setting its affinity

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] tracing/hwlat: start_single_kthread() leaves
  PF_NO_SETAFFINITY set on hwlatd
--

commit 649d99e25afc0a134fd743f085f8409aa8adb366
Author: Wenwu Hou <hwenwur@gmail.com>

tracing/hwlat: Bind the sampler kthread instead of setting its affinity

This commit replaces set_cpus_allowed_ptr() with kthread_bind_mask() in
start_single_kthread() and validates that tracing_cpumask overlaps with
cpu_online_mask before creating the hwlatd kthread.

> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> index 3fe274b84f1c2..65e184d800ff1 100644
> --- a/kernel/trace/trace_hwlat.c
> +++ b/kernel/trace/trace_hwlat.c
> @@ -440,7 +437,14 @@ static int start_single_kthread(struct trace_array *tr)
>  
>  	}
>  
> -	set_cpus_allowed_ptr(kthread, current_mask);
> +	kthread = kthread_create(kthread_fn, NULL, "hwlatd");
> +	if (IS_ERR(kthread)) {
> +		pr_err(BANNER "could not start sampling thread\n");
> +		cpus_read_unlock();
> +		return -ENOMEM;
> +	}
> +
> +	kthread_bind_mask(kthread, current_mask);

[Severity: Medium]
Does leaving PF_NO_SETAFFINITY set on hwlatd prevent userspace from
changing its CPU affinity or moving it into a cgroup?

kthread_bind_mask() sets PF_NO_SETAFFINITY, which kthread() only checks on
initial wake-up before calling kthread_fn().  Because kthread_fn() never
clears the flag, sched_setaffinity() and cgroup_attach_permissions()
reject hwlatd with -EINVAL in both round-robin and none modes.  That also
makes the user-affinity override check in move_to_next_cpu() unreachable
from sched_setaffinity():

kernel/trace/trace_hwlat.c:move_to_next_cpu() {
	/*
	 * If for some reason the user modifies the CPU affinity
	 * of this thread, then stop migrating for the duration
	 * of the current test.
	 */
	if (!cpumask_equal(current_mask, current->cpus_ptr))
		goto change_mode;
    ...
}

Clearing PF_NO_SETAFFINITY in kthread_fn() once the thread has started, as
done in 4998e7fda149 ("tracing/osnoise: Switch from PF_NO_SETAFFINITY to
migrate_disable"), would avoid this regression while still skipping
kthread_affine_node() on the initial wake-up.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111020755.1422284-1-hwenwur@gmail.com?part=1